In a wire-matrix page printer such as that disclosed in U.S. Pat. No. 3,982,633, granted on Sept. 28, 1976, to J. A. Bellino et. al., a carriage-advance stepping motor rotates a lead screw. The lead screw advances a wire-matrix printing carriage from left to right and vice versa across the width of a printing platen in order to print alpha numeric indicia on a sheet of paper. The carriage advance motor of the abovementioned Bellino et. al. patent can be idled in one location, accelerated from rest to a desired continuous speed, and run at that continuous speed to facilitate matrix printing on the paper.
The speed and power level of excitation of the stepping motor is preferably controlled by a power control circuit disclosed in U.S. Pat. No. 4,107,595, issued Aug. 15, 1978, in the name of W. T. Campe and in U.S. Pat. No. 4,107,594, issued Aug. 15, 1978, in the name of N. A. Jacobs. The responsiveness of the matrix printer is directly related to how fast the carriage can be started from rest and accelerated to its operating speed. Acceleration is a function of the power applied to the motor during startup. However, in order to use reasonable power supplies and reasonable operating components, a maximum power must be chosen which is a compromise.
Stepping motors wound for single phase excitation are arranged to energize diametrically-opposite poles of the stator with opposite magnetic polarity such that the flux path of the motor is from one pole, through the rotor, and to the pole on the diametrically-opposite side of the stepping motor. Flux return is through the body of the stator. This is a very efficient form of excitation, but the rotor has a greater tendency to ring or oscillate as it steps from one set of poles (or phase) to the next.
In a stepping motor wound for two-phase excitation, diametrically-opposite poles of the stator comprise a phase coil set and are wound to generate the same magnetic polarity. However, successive phase coil sets on adjacent poles of the stator are wound to produce opposite polarity. Consequently, when two adjacent poles (two successive phase coil sets) are energized in two-phase excitation, the nearest arm of the rotor tends to assume a position midway between the two excited poles. The flux path is not diametrically across the rotor but is in a shorter circuit from one pole face of the stator to the arm of the rotor and returning through the adjacent stator pole, and, of course, through the structure of the stator. Such two-phase excitation has a reputation for being somewhat higher in losses but is known to have a lower level of oscillation as the rotor is stepping from position to position by successive or stepped excitation of the motor.